Airtight construction and a thick layer of exterior foam save energy and reduce the risk of rot and mold

Moisture and Air Quality

In buildings this tight, relying on passive ventilation by natural infiltration or even exhaust-only fans (which require makeup air) would be a mistake. The home would have poor air quality and moisture and condensation problems. So I put a heat-recovery ventilator (HRV) in every house I build. The HRV provides a measured supply of fresh air and recovers much of the heat that would be lost with a simple fan system. HRVs also address the issue of indoor humidity by replacing stale humid air with drier outside air.

Installing the Membrane

Most of the houses I build have wood or vinyl siding; on those jobs we typically install a 6-mil poly barrier membrane, lapping the seams shingle style and sealing the laps with a non-hardening acoustical sealant.

Poly is slippery to walk on and will tear if you drag trusses across it, so when we get to the top of the wall we switch to a strip of 9-inch peel-and-stick flashing. The flashing adheres to the top plate and laps an equal distance onto the poly and the inside face of the plate.

The author’s crew applies a 9-inch-wide peel-and-stick flashing to the top plates, adhering it to the inside edge of the plates.

Lapping it onto the 6-mil poly membrane on the sheathing.

Pencil marks will not show on this material, so we use a white marker to lay out truss locations. Once the trusses are set, we install the ceiling membrane, lapping its edges onto the peel-and-stick flashing and sealing the lap with acoustical sealant.

Installing Windows

Windows can be installed either at the face of the sheathing — in a recess — or out at the face of the wall. From a performance standpoint, a recess is better, because the window is somewhat protected from wind-washing and the interior glass is more easily warmed by the heat in the room. By contrast, windows installed at the face of the wall are in an interior recess, separating them from the warm air inside (especially if a curtain is drawn) and exposing the outer layer of glass to cold wind. I’ve observed that in extremely cold weather — when it’s 25°F below zero, for example — frost tends to form inside windows installed at the face of the wall, whereas frost rarely occurs on inset windows.

Recessed installation. I’ve installed windows both ways, but because of the frost problem I now do only recessed installations. A recessed installation is more complicated because the sides of the recess must be covered with exterior jamb extensions. On vinyl-sided homes, we make the extensions from 20-gauge metal coil stock. The bottom is sloped to shed water, and there are flanges on both edges — an inner flange that gets fastened to the sheathing and an outer flange that laps over the 1x4 strapping that we install on top of the EPS around the window.

When installing recessed windows, the author uses sill and jamb extensions bent from coated sheet metal. The flanges at the outer edges are carefully placed to allow for the thickness of the foam, 3/4-inch strapping, and the vinyl siding, which will tuck underneath.

Note the lines of black acoustical caulk used to seal the jamb extensions to the poly membrane and the poly seams.

We’ve also made extensions from wood and cellular PVC. These solid extension jambs are glued and screwed at the corners and fastened to the wall over a thick bead of sealant. We either toe-screw them to the framing or fasten them from the inside with metal clips.

Window bucks. Because it’s less expensive, many of my past customers chose to have doors and windows installed at the outside face of the wall. We did this by extending the rough openings with bucks ripped from 2-by lumber. The buck fits inside the opening and extends from the inside face of the frame to the outside face of the furring that goes over the foam.

The window is installed in the buck and the fins lapped with peel-and-stick flashings that extend back to the wall membrane. Though this method is less expensive than fabricating jamb extensions, it requires more care with the flashing. From the standpoint of moisture intrusion, I felt comfortable doing it around Fairbanks because we don’t get a lot of rain, but in a wetter climate I would recommend recessing the windows. As I mentioned above, I no longer use this technique because of the icing problem my customers experienced. But for builders in a warmer climate, it could still be a reasonable approach.

On past jobs, the author installed windows at the face of the wall by setting them in solid lumber bucks protected with peel-and-stick flashing.

The bucks are attached to the inside of the rough opening and taped or flashed to the wall membrane.

Bucks are sized so that when foam insulation and furring are installed, the siding will be in the proper plane.

Installing the Exterior Insulation

The EPS insulation can be lightly attached with framing staples or nails because the furring strips, which get screwed through into the studs, will securely hold it in place. We stagger seams and lap corners in successive layers so that air doesn’t have a direct path through to the wall. The boards are butted to the sides of window bucks and jamb extensions, and gaps between sheets and around windows and doors are filled with minimally expanding spray foam.

Some areas have to be insulated and sealed before the framing is complete. For example, where the roof of an unheated attic butts to a sidewall, we’ll insulate that wall before framing the roof.

To ensure the continuity of the barrier membrane and exterior insulation, the wall to the right of this garage roof was covered with poly and foam before the roof was framed.

Moving truss blocks in line with the outer edge of the exterior EPS will allow the cellulose in the attic to completely cover the full thickness of the exterior walls.

We insulate attic gables at least as high as the top of the attic insulation. The area above does not have to be insulated, but it needs to be built out to the plane of the foam below, which we often do with scrap pieces of insulation.

If the inspector and engineer will allow it, we push the truss blocks along the eaves out to the face of the foam so that the blown-in attic insulation will cover the entire top of the wall, including the EPS. Otherwise, we carry the insulation boards up to the top of the attic insulation by fitting them around the rafter tails.

Furring

We provide nailing for siding by installing 1x4 furring or strips of 3/4-inch plywood over the foam, fastening through to the studs with long screws. We space the screws about 12 inches on-center and make sure we penetrate at least 1 1/2 inches into the framing. These long fasteners have to be mail-ordered; we use Wind-Lock W-SIP screws (800/872-5625, wind-lock.com) and FastenMaster HeadLok and OlyLog screws (800/518-3569, fastenmaster.com).

The EPS insulation is first tacked in place, then secured with long screws fastened through furring strips into the framing.

Wide furring members are needed at corners and around openings.

Wide furring members are needed around openings to provide backing for trim.

Screws are very heat-conductive and can cause condensation where they miss the studs. We always check from the inside for missed fasteners, reinstall them, and use spray foam to seal the holes.

Cost and Payback

Though the REMOTE method is more expensive than conventional construction — EPS costs more than fiberglass and there’s extra labor involved — the added costs are offset by reduced energy use and a longer building life cycle. The CCHRC has estimated a three- to five-year payback period from energy savings, and there’s no question that a building free of moisture problems will have a considerably longer life. But what’s equally important is that good insulation, tight construction, and proper heat-recovery ventilation add up to a comfortable, healthy house to live in.

Thorsten Chlupp owns REINA, LLC, in Fairbanks, Alaska. Special thanks to Ilya Benesch of the CCHRC for providing technical advice.